This invention relates to the field of radiography. More particularly, it relates to improvements in apparatus for scan-stimulating an image-storage phosphor to recover a latent image previously formed therein by, for example, an imagewise exposure to x-radiation.
U.S. Pat. No. 3,859,527 (Re. No. 31,847) to G. W. Luckey discloses the use of storage phosphors as a recording medium in radiography. The recording medium is first exposed to x-rays of wavelength .lambda..sub.1 to form a latent image in the phosphor. The incoming flux of x-rays produces a number of excited electrons and holes in the phosphor, some of which are trapped in long-lived (storage) states within the phosphor. At a later time the phosphor may be destructively scanned by stimulating radiation of wavelength .lambda..sub.2 to produce a luminescent emission of wavelength .lambda..sub.3, which is proportional to the original x-ray exposure. The term "destructively" is used herein to denote that the phosphor is discharged by the stimulating radiation, and that only a finite amount of stimulated radiation is emitted by the phosphor, regardless of the quantity of stimulating radiation applied. The terms "storage phosphor(s)" and "phosphor(s)" as used herein, refer to phosphors that, upon stimulation, destructively release emitted radiation.
The above described system uses conventional x-ray exposure equipment. However, in place of the screen and film of conventional radiography a recording medium in the form of a photo-stimulable luminescent storage phosphor is used. After exposure the recording medium is scanned, in a raster pattern by a laser beam deflected by an oscillating or rotating scanning mirror, and the luminescent emission at wavelength .lambda..sub.3 is collected and detected by a photodetector such as a photomultiplier tube and converted to digital information which is transmitted to a computer which, in turn, processes the image. U.S. Pat. No. 4,778,995 to R. W. Kulpinski et. al. discloses in schematic form the basic method of scanning, in which the laser is fixed, the laser beam is deflected in the fast or line scan direction (in this case by a rotating polygon mirror), and the recording medium advanced in the slow or page scan direction by a suitable sheet drive mechanism.
Though not used for scanning phosphors, scanning systems other than the type described in U.S. Pat. No. 4,778,995 are known. Stroke marking, used for both laser engraving (evaporating material up to 0.25 mm deep) and annealing (causing a color change by localized instantaneous heating of material without removing material from the surface) utilizes a fixed laser, a microprocessor controlled dual galvanometer system, and flat field focusing optics. The beam is deflected by galvanometer mirrors in both the x and y directions before it is focused onto the workpiece. Scanning systems are also known in which the flat field optics, such as used in stroke marking, have been eliminated. In an x-y laser printing system disclosed by General Scanning, Watertown, Mass., the flat field optics have been eliminated by modifying the x and y focus as the beam moves across the image plane by using a "dynamic focusing telescope" consisting of two moving focusing lenses. The slow scan or y focus is corrected by a lens which is adjusted in steps as the beam moves down the image plane. The fast scan or x focus is accomplished with what is identified as a " resonant lens mover". In both of the above described systems, the image plane remains stationary. It is not known to use the above described systems for scanning stimulable storage phosphors whether turbid or transparent.
Currently, optically turbid (non transparent) storage phosphors are used as the recording media in digital radiography systems. However, there are certain advantages that an optically transparent phosphor has over a turbid phosphor. Since the MTF (Modulation Transfer Function; a measure of the ability of the system to record details) of the transparent phosphor imaging system is limited mainly by the effective size of the scanning beam of stimulating radiation, which may be adjusted to a desired size, the MTF may be made much higher than in a comparable turbid phosphor system. In addition, the x-ray absorption of the sheet may be increased by making it thicker, without increasing the effective size of the scanning beam. In this way the signal-to-noise ratio of the x-ray detector may be improved. In the conventional turbid storage phosphor sheets, the thickness is limited by the spreading of the scanning beam in the turbid phosphor. Optically transparent phosphors are disclosed in U.S. Pat. No. 4,733,090 to C. D. DeBoer et. al.
In the practice of radiography, the distance between the x-ray tube and the recording medium is limited. The intensity of the x-ray flux decreases with the square of the distance from the tube. Thus, to maintain a constant x-ray exposure at the recording medium, as the distance is increased, the tube current or exposure time or both must be increased. However, the current cannot be increased indefinitely because of the resultant heating of the tube anode. The tube-to-recording medium distances in use typically range from 50 to 150 cm, depending on the type of examination, exposure time and technique used.
The combination of the high resolution (or MTF) and the thickness of an optically transparent phosphor creates a new problem, namely, resultant images are blurred due to: (1) the obliqueness of the x-rays, which form the edge of the shadow of an object, relative to the surface of the phosphor, even if the source of x-rays (i.e., the tube anode) is a point; and (2) the fact that the direction of the radiation used to scan the phosphor is not the same as the direction of the x-radiation. With reference to FIGS. 1-3, L is the anode to transparent phosphor distance; t, the thickness of the phosphor 13; x, the distance from the centerline 3 at which a particular x-ray strikes phosphor 13; and .DELTA.X, the amount of blurring of a line 5 in thin opaque sheet 7. As an example, if L=80 cm, x=8 cm and t=2 mm, then: EQU .DELTA.X=(x/L) t=200 .mu.m
This is obviously large, compared to the diameter of the scanning beam, which may be 100 .mu.m or less.
While C. D. DeBoer et. al. recognized that ideally the beam of the scanning laser should follow the path of the x-radiation, they suggested no solution to the above described problem. Conventional scanning in a raster pattern does not solve the problem. The object of the present invention is to solve the above described problem.